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參數(shù)資料
| 型號： | MAX3865EGJ |
| 廠商： | MAXIM INTEGRATED PRODUCTS INC |
| 元件分類： | 其它接口 |
| 英文描述： | 2.5Gbps Laser Driver with Automatic Modulation Control |
| 中文描述： | SPECIALTY INTERFACE CIRCUIT, CQCC32 |
| 封裝： | 5 X 5 MM, 0.90 MM HEIGHT, QFN-32 |
| 文件頁數(shù)： | 9/16頁 |
| 文件大小： | 633K |
| 代理商： | MAX3865EGJ |
Laser Current Requirements
Bias and modulation current requirements can be
determined from the laser threshold current and slope
efficiency. The modulation and bias currents under a
single operating condition are:
For DC-coupled laser diodes:
I
BIAS
> I
TH
where I
TH
is the laser threshold current.
For AC-coupled laser diodes:
Given the desired parameters for operation of the laser
diode, the programming of the MAX3865 is explained
in the following text.
Current Limits
To keep the modulation current in compliance with the
programmed value, the following constraint on the total
modulation current must be made:
DC-Coupled Laser Diodes:
V
CC
- V
DIODE
- I
MOD
(R
D
+ R
L
) - I
BIAS
R
L
≥
1.8V
For V
DIODE
—
Laser diode bias point voltage
(1.2V typ)
RL
—
Laser diode bias-point resistance (5
typ)
RD
—
Series matching resistor (15
typ)
AC-Coupled Laser Diodes:
To allow larger modulation current, the laser can be
AC-coupled to the MAX3865 as shown in the
Typical
Application Circuit
. In this configuration, a constant cur-
rent is supplied from the inductor L
P
. The requirement
for compliance in the AC-coupled circuit is as follows:
I
CC
×
2
The AC-coupling capacitor and bias inductor form a
second-order high-pass circuit. Pattern-dependent jitter
results from the low-frequency cutoff of this high-pass
circuit. To prevent ringing:
For deviation from 50% duty cycle or for runs of con-
secutive identical digits (CID), the low-frequency corner
formed by the LC circuit must be low enough to limit
the droop.
If droop = 6.7%, number_CID = 100 and data_rate =
2.5Gbps, then possible values for L
P
and C may be
L
P
= 6μH and C = 0.056μF. Both L and C must be
increased in value to reduce droop without ringing.
Programming the Maximum Bias Current
In AMC (or APC) mode, the bias current needs a limit if
the loop becomes open. R
BIASMAX
sets the maximum
allowed bias current. The bias current is proportional to
the current through R
BIASMAX
. An internal current regu-
lator maintains the band-gap voltage of 1.2V across the
programming resistors. Select the maximum I
BIAS
pro-
gramming resistor as follows:
Alternatively, a current DAC forcing I
DAC
from the
BIASMAX pin may set the current maximum:
I
BIASMAX
= 480
I
DAC
When the AMC or APC loop is enabled, the actual bias
current is reduced below the maximum value to main-
tain a constant average current from the monitor diode.
With closed-loop control, the bias current will be deter-
mined by the transfer function of the monitor diode to
laser-diode current. For example, if the transfer function
to the monitor diode is 10.0μA/mA, then setting I
MD
for
500μA will result in I
BIAS
equal to 50mA.
In manual mode, the bias current I
BIAS
is I
BIASMAX
as
set by R
BIASMAX
.
Programming the Average Monitor
Diode-Current Set Point
The APCSET pin controls the set point for the average
monitor diode current when in AMC or APC mode. The
APCSET current is externally established in the same
manner as the BIASMAX pin. The average monitor
diode current I
MD
can be programmed with a resistor
as follows:
Alternatively, a current DAC at the APCSET pin can set
the monitor diode current by:
average I
MD
= 5
I
DAC
average I
V
R
k
MD
APCSET
_
.
=
×
+
5
1 2
2
I
V
R
k
BIASMAX
BIASMAX
=
×
+
480
1 2
.
2
Droop
Number CID
Data Rate
_
L
C
P
=
×
×
_
R
(
R
L
C
D
L
P
+
)
≥ ×
2
V
R
(
R
V
MOD
D
L
+
)
≥
1 8
I
I
I
BIAS
TH
MOD
2
>
+
I
P
r
r
MOD
AVG
η
e
e
= ×
2
×
+
1
1
M
2.5Gbps Laser Driver with Automatic
Modulation Control
_______________________________________________________________________________________
9
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